How do you know when the dealer offer a good trade in price and im not getting over charge on my purchase

I look up trade in before I trade. Both NADA and Kelly Bluebook have listings for Trade In Value. Also tell you Trade In Value is starting point for negotiating deals.

It is hard to know what a trade is actually worth. Those forums and sites are good guides. A lot depends on the market, or area that you are in and what similar models are currently being sold for. Read this article for a more in depth discussion. http://carsandtrucksbyjody.wordpress.com/about/